1. Field of the Invention
This invention relates to remotely powering wireless devices. More particularly, this invention relates to minimizing heating of body tissues during exposure to an electromagnetic field while powering a wireless medical device.
2. Description of the Related Art
Implantable or insertable medical devices are sometimes wirelessly powered by the transmission of radiofrequency (RF) energy from a radiator that is located external to a patient's body. One or more power coils incorporated in the device receive the radiofrequency energy. For example, some wireless location transponders comprise such power coils, and one or more position sensing coils for receiving externally generated position signals. The transponders typically use the wirelessly received energy to convert the position signals into high frequency signals, and to drive the power coil (or a separate transmission coil) to transmit the high frequency signals to an externally located processing unit, which responsively determines the position and the orientation of the transponder.
For example, a transponder and apparatus for operating the transponder employing analog high-frequency signals is described in U.S. Patent Application Publication No. 2003/0120150, entitled “Wireless Position Sensor,” whose disclosure is herein incorporated by reference. The apparatus for operating the transponder includes a plurality of field generators, which generate electromagnetic fields at different respective frequencies in a vicinity of the object, and a radiofrequency driver, which radiates a radiofrequency driving field toward a wireless transponder. The transponder includes at least one sensor coil, in which a signal current flows responsively to the electromagnetic fields, and a power coil, which receives the radiofrequency driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal for communicating information to a receiver or interrogator. In medical applications such transponders, whether analog or digital, typically comprise multiple sensor coils, such as three mutually-orthogonal coils, as described in European Patent EP 0 776 176 to Ben-Haim et al. Position and orientation coordinates of the transponder can thus be determined without ambiguity.
These location transponders enable the determination of the position and orientation of an object in the body without the need for any wired connection between the sensing coil and the external processing unit. Such wireless transponders may be implanted in the body of a patient, such as in a bone of the patient, or incorporated into an implantable medical device. However, there is a concern that when the device is being actively powered by a radiofrequency driver, there could be harmful local tissue heating resulting from non-uniformities in the electromagnetic field.
In general the deposition of radiofrequency energy in the human body tends to increase the body temperature. A World Health Organization document, Environmental Health Criteria 137, available on the Internet at the URL “http://www.inchem.org/documents/ehc/ehc/ehc137.htm”, indicates that there exists a threshold specific absorption rate (SAR) of radiofrequency energy for frequencies above about 1 MHz of 1-4 W/kg, above which there is increasing likelihood of adverse health effects. Below about one MHz, standards are based on induced currents in the body, causing shocks and burns. Furthermore, pulsed fields may be of particular concern. In the case of pulsed electromagnetic fields, it has been shown, under a number of conditions, that the thresholds for biological effects at frequencies above several hundred MHz are decreased when the energy is delivered in short (1-10 μs) pulses. A safe limit for such pulses cannot even be identified on the basis of available evidence. It would appear to be prudent to minimize exposure of patients and medical personnel to such fields.